1. Field of the Invention
The present invention relates to a paging processing system in a virtual storage device enabling prevention of reduction in throughput of a computer by reducing a time required for paging to be conducted in large-scale calculation and a paging processing method thereof.
2. Description of the Related Art
In a conventional computer having a virtual storage device, it is possible to virtually assume and use memory space larger than a physical memory mounted on the computer. The mechanism is compressing useless memory space by allocating only the memory space being actually used out of virtual memory space to memory space of the physical memory.
Allocation from virtual memory space to memory space of a physical memory is conducted on the basis of a page which is a small unit obtained by dividing each memory space. In other words, out of virtual pages, a page being actually used is allocated to a physical page.
In this system, however, a physical memory might run out of pages when numbers of programs are executed simultaneously or when a program requiring enormous memory space is run.
Therefore, so-called page-out processing is conducted which is to once write a page that has not been used for the longest time at a certain point of time to an external storage device to free a page of a physical memory and allocate the page to a page that is to be used at the current time point to carry out execution.
Thereafter, when there arises a need of reference/modification of data of the paged out page, so-called page-in processing is conducted which is to read, if a free physical page exists, the page data retained in the external storage device into the physical page and re-allocate the physical page to a virtual page. If there exists no free physical page, another page is once paged out to make a free page in the physical memory and conduct page-in processing.
Conventional virtual storage control device is disclosed, for example, in Japanese Patent Laying-open (Kokai) No. Showa 57-24081. Proposed in the virtual storage control device recited in Japanese Patent Laying-Open No. 57-24081 is a technique of conducting data page-in/page-out from/to an external storage device as mentioned above.
In large-scale numerical computation processing or simultaneous running of numbers of programs, a virtual storage device needs more pages than those of a mounted physical memory to require page-in/page-out processing to be conducted frequently. Numerical simulation such as the finite element method, for example, requires an enormous array area to cause a shortage of a storage region, so that paging to an external storage device occurs frequently.
With conventional techniques, however, page-in/page-out processing consumes so much time that occurrence of many times of paging processing will considerably reduce throughput of the entire computer.
The reason is as follows. In conventional techniques, when a real storage region fails to accommodate a virtual storage region, part of data in the real storage region is temporarily saved in an external storage device to make a free area and the virtual storage region is allocated to the free area, and the data saved in the external storage device is returned to the real storage region as required. Although a magnetic disk or a magnetic drum device is used as the external storage device responsible for the data writing/reading, a rate of data writing/reading to/from these storage devices is extremely low as compared with a rate of data writing/reading to/from a memory and much time will be accordingly consumed for paging processing.